Сraig. Dental Materials

Once good wetting is achieved, the adhesive should intimately contact the substrate to produce physical, chemical, or mechanical bonding. For effective chemical bonding, the distance between the adhesive and substrate must be less than a few Angstroms and a high density of new bonds must form along the interface. Because this is rarely the case, bonding of restorative materials involves mostly mechanical bonding. Mechanical bonds (gross mechanical retention and micro-mechanical retention) involve adhesive interlocking with surface irregularities. Cavity preparation produces some irregularities. In other cases, surface roughness is increased by sandblasting or etching.

The final practical consideration for bonding is the method of curing (polymerizing) the adhesive. Most contemporary bonding agents harden by chemical reactions initiated by visible light, although self-cured and dual-cured systems are also available. If curing does not continue to

a sufficient degree, the under-cured adhesive may not provide good retention and sealing.

MECHANISMS OF INTERFACIAL DEBONDING

Debonding of dental joints occurs by a process of crack formation and propagation and subsequent joint failure. Cracks form at defects along the interface. Examples of defects include sites of interfacial contamination, excess moisture, trapped air bubbles, voids formed during solvent evaporation, places of poor wetting, bubbles within the adhesive, and curing shrinkage pores (Fig. 10-3). The bonded system includes the outermost layers of the substrate, which may have been altered during bonding techniques, the adhesive layer, and the restorative material interface. For all practical purposes, the bulk properties of the tooth substrates (enamel, dentin) and restorative substrates (composites, ceramics) are much stronger than the bond strength of

composite Enamel-adhesive-

composite

the restorations (Table 10-1). Therefore cracks that form generally remain in the bonded interface zone.

As cracks grow, they contribute to stress concentrations or stress redistributions within the substrates. The final failure may often extend for short distances through portions of tooth structure or restorative material. The amount of substrate fractured is often indicative of the relative strength of the joint. Failed surfaces should be examined carefully with moderate magnification to identify the origin of the critical crack, although this crack may not be readily apparent. Therefore failures are reported as being within the substrate (cohesive), between the adhesive and substrate (adhesive), within the restorative material (cohesive), or mixed. Generally, this information is not very useful to understanding why the failure occurred.

Bond strength testing is one of most popular analyses conducted in evaluations of dental ma-

terials. Available tests accommodate differences in specimen size, test configurations for loading, and patterns of loading. There is no absolute bond strength; rather the measured bond strength is influenced by the concentrations of defects created in forming the interface and experimental testing variables, which may or may not be controlled. Different research investigators and different testing designs produce different values. Although the ADA and IS0 have attempted to standardize the procedures, different values of bond strength are measured in different laboratories. It is therefore critical to include a control along with test results. Generally, the control for bond strength to dentin is the bond strength to enamel.

Bond strength can be studied using prospective or retrospective clinical models, or with in vitro using simulated clinical models or bonding to a standardized substrate. In a simulated clinical model, for example, a ceramic restoration is bonded with resin cement to extracted human or bovine teeth, and in vitro bond strength is tested in shear or tension. The advantage of this model is that both the tooth and ceramic restoration are present, so the test appears to be clinically relevant. The disadvantage is that often bond failures occur at both interfaces, so it is difficult to isolate the weak link in the tooth-resin cementceramic system. A more fundamental test is the isolated interface model, in which, for example, bonding of the tooth-resin cement interface is studied separately from the ceramic-resin cement interface.

Shear bond strength is the most prevalent bond test (Fig. 10-4). It is difficult to control the position of the knife edge in this test, so there is some bending involved that causes variability. A reproducible tensile bond test for the isolated interface model is the inverted, truncated cone test (Fig. 10-5). Diameters of bonded test interfaces typically range from 3.0 to 4.5 mm. It appears that tests that use 3-mm joints produce less variation in bond strength. Shear and tensile tests report bond strengths of clinically successful composites bonded to human enamel and dentin to be in the range of 15 to 35 MPa. Coefficients of

Fig. 10-4 Composite bonded to dentin within plastic rings and debonded by shear bond strength (SBS) testing along the interface.

(Courtesy SC Bayne, Universiv of North Carolina School of

Dentistry, Chapel Hill, NC.)

Screw

Assembly

Cement

(part A)

Specimen

Attached to testing

machine II

1

I I

Fig. 10-5Tensile bond strength test using inverted, truncated cone in an isolated interface test model.

(Adapted from Barakat MM, Powers JM: Aust Dent J

31:4l5, 1986.)

variation for shear bond tests range from 20% to 60%, whereas those for tensile bond strength tests range from 20% to 40%.

During the mid 1990s, there were numerous attempts to minimize the problems of in vitro bond strength testing and allow testing with fewer numbers of extracted teeth. The microtensile bond strength test (Fig. 10-6) was devised to be a more clinically relevant test. It is claimed that the test reduces the probability of crack initiation and propagation within individual specimens because of the small bonded area (1 mm2).

Adapted from LeGeros RZ: Calcium phosphates in oral biology and medicine, Monogr Oral Sci 15:108-113,1991.

For all bond strength tests, faster loading rates tend to increase the observed bond strengths. Higher test temperatures may contribute to postcuring and strengthen the adhesive. Wet specimens are generally weaker than dry ones, as a result of the plasticizing effect of absorbed water. Fig. 10-7 demonstrates the variability reported by different laboratories attempting to test the same bonding agent.

CHARACTERIZATION OF H

ENAMEL AND DENTIN

Adhesion in dentistry involves a wide range of substrates, but most applications involve adhesion to enamel and dentin. Challenges for adhesive procedures are related directly to the structure of these tissues. The following information is therefore provided as background for bonding to enamel and dentin.

STRUCTURE AND MORPHOLOGY

OF ENAMEL

Enamel prisms are filled with millions of hydroxyapatite crystals, which occupy about 89% by volume of the entire enamel structure. Between the crystals are small amounts or remnant

(Courtesy B Rosa, Sao Paulo, Brazil.)

protein structure and water. The general composition of enamel and dentin by weight and volume are reported in Table 10-2. Enamel crystals are packed with their long axes roughly aligned with the long axis of the enamel prism. Crystals are hexagonal in cross-section and elongated. Looking down along the long axis of the enamel prism, the crystals are perpendicular to the view in the center of the prism but are more tipped toward the periphery. The crystals in the tail are the least densely packed and tipped at perhaps as much as 30 degrees away from perpendicular. Chemical properties of the prism vary between the center and perimeter.

Enamel prisms are essentially parallel to each other and run from the dento-enamel junction (DEJ) outward in a radial pattern. In the neighborhood of enamel cusps, the prisms are perpendicular to the DEJ. Near the cemento-enamel junction, the prisms are highly tipped. It is crucial to avoid undermining enamel rods during cavity preparation, or bonding will tend to dislodge the

Shear bond strength (MPa i- SD)

Fig. 10-7 Example of variability of shear bond strengths reported by 12 different laboratories performing the same test on 3M Single-Bond Adhesive and 3M Zl00Composite.

(Adapted from May KN Jr, Swift EJ Jr, Bayne SC: Am J Dent 10:195, 1997.)

enamel prisms in those regions during mechanical loading.

Etching of enamel with phosphoric acid eliminates smear layers associated with cavity preparation, dissolves persisting layers of prismless enamel in deciduous teeth, and differentially dissolves enamel crystals in each prism. The pattern of etching (Fig. 10-8) is categorized as Type 1 (preferential prism center etching), Type 2 (preferential prism periphery etching), and Type 3 (mixed). There appears to be no difference in micro-mechanical bonding of the different etching patterns. In a standard cavity preparation for a composite, the orientation of the enamel surfaces being etched could be perpendicular to enamel prisms (perimeter of the cavity outline), oblique cross-section of the prisms (beveled occlusal or proximal margins), and axial walls of the prisms (cavity preparation walls).

During the early stages of etching, when only a small amount of enamel crystal dissolution occurs, it may be difficult or impossible to detect the extent of the process. However, as the etching pattern begins to develop, the etched surface develops a frosty appearance (Fig. 10-9, B),

which has been used as the traditional clinical landmark for sufficient etching.

STRUCTURE AND MORPHOLOGY OF DENTIN

Dentin's composition is much more rich in organic material than enamel. Only about 50% by volume of dentin is mineralized with hydroxyapatite crystals (see Table 10-2). A large proportion of the crystals is interspersed among collagen fibers. Within collagen fibers, individual fibrils contain crystals at their ends as well. The lower mineral content of dentin permits more elastic deformation during loading.

Dentin tubules are 0.5 to 1 5 pm in diameter and are formed at a slight angle to the DEJ and pulp chamber. There is a higher density of tubules along the inner or deep dentin (43,000 tubules/mm2) than at the middle dentin (35,000 tubules/mm2) or the outer, superficial dentin (15,000 tubules/mm2). A labyrinth of secondary tubules among neighboring tubules often interconnects primary tubules. Fig. 10-10 is a scanning electron micrograph of dentin prepared for bonding. It reveals the primary tubule in the

center with secondary (lateral) tubules branching off from the tubule. Intertubular dentin comprises collagen, hydroxyapatite, and water. Under normal circumstances, the tubule is filled with the odontoblastic process.

Etching of dentin surfaces primarily dissolves hydroxyapatite crystals within the surface of the intertubular dentin and along the surface of the outermost peritubular dentin. A smear layer exists from cavity preparation (Fig. 10-11, A and B) that is typically 1 to 2 pm thick with smear plugs

within the ends of the tubules (Fig. 10-11, 0.The smear layer is almost entirely hydroxyapatite debris from high-speed cutting during cavity preparation. High surface temperatures along the cutting interface pyrolize most of the organic material.

When the etchant first contacts the smear layer it begins to dissolve and penetrate it. Immediately the underlying intertubular dentin, peritubular dentin, and tubules come into contact with acid. Intertubular dentin contains

about 50% hydroxyapatite crystals; these crystals are the same size as in enamel and are embedded within the spaces between collagen fibers. This material is relatively quickly dissolved. Peritubular dentin is about 80% to 90% hydroxyapatite crystals by volume and is readily dissolved in acid. The dentinal fluid is highly buffered. Therefore acid that mixes with dentinal fluid is quickly neutralized. Effects of acid etching are typically limited to 0.1 to 5 pm of

the superficial region of intertubular dentin and about 2 to 10 pm along the walls of peritubular dentin (Fig. 10-12, A). The result of the etching process is the creation of a demineralized zone of dentin between the tubules and along the outer mouth of individual dentinal tubules. This surface is porous and allows primer penetration for tag formation. Fig. 10-12, B, is an atomic force microscope image of the etching sequence.

ENAMEL AND DENTIN BON

AGENTS FOR DIRECT C W

OVERVIEW

A chronology of the development of bonding agents used to place direct composites is shown in Table 10-3. Modern bonding agents contain three major ingredients (etchant, primer, adhesive) that may be packaged separately or combined. Examples of commercial bonding agents are shown in Fig. 10-13. Table 10-4 presents a

summary of the components of currently available fourth-, fifth-, and sixth-generation bonding agents. Typical compositions of these components are summarized in Table 10-5.

Etchants are relatively strong acid solutions that are mainly based on phosphoric acid. Primers contain hydrophilic monomers to produce good wetting. Adhesives include typical dimethacrylate oligomers that are found in composites. Most fourthand fifth-generation bonding agents are remarkably similar.

COMPOSITION

Etchants A wide range of organic (maleic, tartaric, citric, EDTA, acidic monomers), polymeric (polyacrylic acid), and mineral (hydrochloric, nitric, hydrofluoric) acids have been investigated as etchants, but phosphoric acid solutions and gels (37%, 35%, 10%) have been shown to produce the most reliable etching patterns. Acid etchants are also called conditioners to disguise the fact that most are relatively strong acids ( p H d .0).

Originally, etching solutions were freeflowing liquids and were difficult to control during placement. Gel etchants were developed by adding small amounts of microfiller or cellulose thickening agents. These gels flow under slight pressure but do not flow under their own weight.

Primers Primers are hydrophilic monomers (see Table 10-5) usually carried in a solvent. Acidic primers containing carboxylic acid groups are used in self-etching bonding agents. The solvents used in primers are acetone, ethanolwater, or primarily water. In some primers, the

Fig. 10-13Examples of dentin bonding agents (circa 2001). A, 4th-generation (Scotchbond Multi Purpose, left), 5th-generation (OptiBond Solo right). B, 5thgeneration (Prime & Bond NT, Single Bond, One-Step). C, 6th-generation (Prompt L-Pop, Clearfil SE Bond).